Parallel flow:

Parallel flow, as described in Figure, exists while both the tube side fluid and the shell side fluid flow in the similar direction. In that case, the two fluids enter the heat exchanger from the similar end along with a huge temperature difference. Since the fluids transfer heat, hotter to cooler then the temperatures of the two fluids approach every other. Remember that the hottest cold-fluid temperature is minimum than the coldest hot-fluid temperature.

Figure: Parallel Flow Heat Exchanger